It has been said that the history of science is the history of scientific instruments; that advances come when the potentials of new instruments are embraced to look in new directions. During the last two years here at SLAC, the E157/E162 Collaboration has broken new ground in the quest to probe nature’s smallest scales by accelerating and focusing electrons in a fundamentally new way. These experiments have used electrostatic waves in “plasmas,” the ionized gases found in fluorescent light tubes, to give particles from the SLAC two-mile linac an extra kick to higher energy while simultaneously focusing them as a collimated beam. So energetic is this plasma-wave acceleration process, that this method may someday enable us to reach energies a hundred or more times higher than the existing linac, but in the same length. These exciting experiments have been mounted by a diverse group of accelerator, laser, and plasma researchers.
Although the concept of a “plasma-wave accelerator” has been around for over 20 years and small-scale experiments have demonstrated the principle in short, millimeter and centimeter-long plasmas, the E157 experiment is the first to use long plasmas to obtain energy gains of interest to accelerator builders. The acceleration chamber containing the plasma is in fact the same length as a typical fluorescent light tube, about 1 1/2 meters. The energy increase per meter of electrons in these first experiments has already been measured to be about five to ten times higher than in the copper accelerator cavities of the SLAC linac. Unlike metallic cavities that suffer electric breakdown (sparking) at high fields, plasmas have no corresponding field limit since they are already ionized.
The plasma acceleration process explored in E157 begins with the 28 GeV electron beam from the linac. Roughly one millimeter-long bunches, each containing about twenty billion electrons, are delivered through the beam switchyard at a rate of 1 or 10 Hz to the Final Focus Test Beam Facility (FFTB). “We use SLAC’s high quality beam to both power the plasma accelerating wave and provide a few test particles to get accelerated by it,” said Mark Hogan. The “plasma accelerator cell” is a 1.4 meter-long oven containing lithium vapor at 3x1015 atoms per cubic centimeter, which was installed in a drift space of the FFTB transport line. To create the plasma, an ultraviolet laser pulse of 100 millijoules per square centimeter is used to ionize a one millimeterwide column lengthwise through the lithium vapor just prior to the bunch’s arrival.
Plasma densities of about 2x1014 electron-ion pairs per cubic centimeter are used in these experiments. The key to particle acceleration in a plasma is to produce a charge separation of the positively-charged lithium ions and negative electrons, and hence a local electric field, which travels as a wave through the plasma. In E157 the method for generating a traveling electrostatic wave in the plasma is analogous to a boat creating a wake in the water. The incoming electron bunch repels the plasma electrons since they have the same negative charge. The heavy lithium ions remain and provide a focusing force to the beam. The plasma electrons later try to flow back in where the ions are, just as water rushes in behind a moving boat. The water in the wake then sloshes back and forth in waves following the boat. Particles making up the tail of the bunch experience the plasma wake and “surf” on the strong electric fields, gaining energy very quickly. As Patrick Muggli, from USC, commented “I always thought of a 30 GeV beam as being really stiff and difficult to move, but the fields in these plasmas are so tremendous that the SLAC beam can be significantly accelerated and even pushed sideways in just centimeters!”
Precise diagnostic techniques were essential for measuring plasma effects on beam particles. To determine the energy gained by electrons from the plasma wake, a dipole magnet was used to bend the beam after the plasma region. Higher energy particles are bent less by the dipole than lower energy ones, and the beam ends up being spread transversely according to energy. Passing this spread bunch through an aerogel cell, the emitted Cerenkov radiation was then time- resolved in a “streak camera” in which light is recorded along a charge-coupled device (CCD) used in digital cameras according to when it arrives.
Transverse focusing of the beam is determined by measuring the beam profile before and after the plasma. To do this, the radiant spot of optical transition radiation (OTR), caused by the beam intercepting thin titanium foils inserted in the beamline, is imaged. The beams are only about a tenth of a millimeter wide requiring high-resolution imaging, and Figure 1 (left) shows the variations in transverse beam size seen in the experiment. It also shows the 16 picosecond “streak” images of the bunch in which early light from the bunch’s head is at left and light from the tail is on the right. When the plasma is on at resonant density, a few tail particles, which gained about 125 MeV, are seen above the lower energy beam core.
The dramatic plasma acceleration and focusing effects seen in electron beams encouraged the collaboration to propose a new experiment, E162, to study the plasma acceleration and focusing of positrons (the antimatter of electrons, which are used in electronpositron colliders). To observe the more subtle effects produced by the positron beam–plasma interaction, the plasma was moved roughly 20 meters upstream to the focal point of the FFTB. The improved magnetic optics at the focal point will allow the collaboration to study matched beam propagation through a long plasma and build an imaging spectrometer to improve the acceleration measurements. The new experiment was approved and is performing three runs at the FFTB during the spring through fall of 2001. Figure 2 (above) shows a preliminary E162 finding of plasma focusing of positrons in the 1.4 meter-long lithium plasma. These results follow the first-ever observation of plasma focusing of positrons reported by the E150 Plasma Lens Collaboration last year in three millimeter-long, hydrogen and nitrogen plasmas (see The Interaction Point, June 2000).
It is hoped that novel particle acceleration and focusing techniques such as these will continue to open new windows for us on nature’s intricate workings at the subatomic scale. The development of high-energy particle accelerators requires the talents and contributions of many people. The members of the E157 and E162 Collaborations express their sincere gratitude to the entire SLAC workforce whose daily efforts have helped make these experiments successful.
For more information about this experiment, check out the E-162 photo tour.